1. Field of the Invention
The present invention relates to a pressure detection device used for detecting a pressure increase in a hydraulic apparatus, more particularly, to a pressure detection device in which a functional material changing an electric resistance corresponding to a load pressure operated thereto is provided between two electrodes thereof.
2. Description of the Related Art
In a hydraulic power steering apparatus used in an automobile or the like, a hydraulic pump is generally driven by an engine of the automobile, and operation fluid discharged from the hydraulic pump is supplied to the power steering apparatus, so as to assist the driver's steering operation.
In such a hydraulic power steering apparatus, when a steering operation is carried out while the automobile is stopped, i.e., the engine is running at its idle rotational speed (hereinafter referred to as "stationary steering operation"), the load pressure in the hydraulic pump increases, whereby the engine may be stalled.
In order to solve this problem, the conventional automobile employs a so-called idle-up mechanism in which a pressure switch is provided for detecting an increase in the load pressure of the hydraulic pump so as to operate the idle-up mechanism while the engine is running at the idle rotational speed.
FIG. 1 shows an example of such a pressure switch, as disclosed in Japanese Patent Laid Open Publication No. 9-147708. In FIG. 1, numeral 1 denotes a switch housing, which is attached to a pressure introduction passage 3 formed in a pump housing. In the switch housing 1, a piston 4 is disposed so as to be axially movable according to an increase of the load pressure led from the pressure introduction passage 3. In an upper portion of the piston 4, a terminal 5 is arranged to be defined as one of a pair of electrode contacts, which is supported by a support ring 7 via a resin 6 serving as an electric insulator.
Between the terminal 5 and the piston 4, a disc spring 8 is provided to be defined as the other electrode contact, which comes into contact with the terminal 5 when being deformed by the pressure force of the piston 4. The disc spring 8 is supported by the support ring 7 and the switch housing 1, and is grounded by electrically communicating from the switch housing 1 to the pump housing 2.
In the pressure switch as configured above, when the load pressure in the hydraulic pump is raised due to the stationary steering operation while the engine is running at the idle rotational speed, the pressure increase acts onto the piston 4, whereby the piston 4 is slid against the spring force of the disc spring 8. With this slide movement of the piston 4, the disc spring 8 is deformed and then is electrically brought into contact with the terminal 5, so that the pump housing 1 is electrically communicated with the terminal 5, whereby the pressure switch generates an electric signal for the idle-up operation to a control apparatus.
However, the conventional pressure switch has such a function that operates in an ON-OFF manner where it is turned on when the load pressure of the hydraulic pump exceeds a predetermined pressure. Therefore, it is impossible to continuously control the engine so as to change the rotational speed thereof according to the load pressure of the hydraulic pump. In the automobile providing such a pressure switch, the engine is controlled so as to increase the idle rotational speed in correspondence with a maximum load of the hydraulic pump when the ON-operation of the pressure switch is carried out, so that the energy consumption of the automobile is increased.
In the hydraulic power steering apparatus, it may be thought to use a pressure sensor to continuously change the idle rotational speed of the engine according to the load pressure of the hydraulic pump. However, the pressure sensor has a complex construction and is expensive compared with the pressure switch as described above. Further, an element consisting of the pressure sensor has a low characteristic in linearity, so that it requires an electric circuit for compensating the output thereof, whereby it is more expensive. This is a reason why the pressure sensor is not used generally.